<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <div id="one"></div>
    <div id="tow"></div>
    <div id="three"></div>
</body>
<style>
    #one{
        width: 100px;
        height: 100px;
        background-color: red;

        float: left;
    }

    #tow{
        width: 100px;
        height: 100px;
        background-color: blue;

        float:right;
    }

    #three{
        width: 100px;
        height: 100px;
        background-color: green;

        float:right;
    }

    #three{
        /* 清除浮动两边 left  right both */
        clear:both; 
    }
</style>
</html>